1. Field of the Invention
This invention relates to a single phase article and to a multiphase composite and to a method for producing the same.
2. Description of the Prior Art
Composite products having multiphases of matrix metal and a hardening phase are used in various applications requiring hard, wear-resistant properties. The composites comprise a metal matrix, which may be for example, iron, nickel, or cobalt, with a hard-phase nonmetallic dispersion therein of, for example, carbides, nitrides, oxynitrides or industrial diamonds.
Tungsten carbide-cobalt composites are one significant example of composites of this type and the production thereof typifies the conventional practices used for the manufacture of these composites.
The manufacturing process consists of synthesis of the pure carbide and metal powders, blending of the carbide and metal powders to form a composite powder, consolidation of the composite powder to produce a "green" compact of intermediate density and, finally, liquid phase sintering of the compact to achieve substantially full density.
Preparation of the tungsten carbide powder conventionally comprises heating a metallic tungsten powder with a source of carbon, such as carbon black, in a vacuum at temperatures on the order of 1350.degree. C. to 1600.degree. C. The resulting coarse tungsten carbide product is crushed and milled to the desired particle size distribution, as by conventional ball milling, high energy vibratory milling or attritor milling. The tungsten carbide powders so produced are then mixed with coarse cobalt powder typically within the size range of 40 to 50 microns. The cobalt powders are obtained for example by the hydrogen reduction of cobalt oxide at temperatures of about 800.degree. C. Ball milling is employed to obtain an intimate mixing of the powders and a thorough coating of the tungsten carbide particles with cobalt prior to initial consolidation to form an intermediate density compact.
Begg et al, U.S. Pat. No. 4,749,545, relates to milling metal composites that contain at least 40% v/v of hard material and states that prior art mixing techniques fail to incorporate as high a proportion of hard material into the composite. However, in Begg the hard-phase particle sizes are in excess of 0.1 micron. Mechanical milling, as used in Begg to reduce the particle sizes of the hard phase, typically produces particles with diameters in excess of 0.1 micron.
Milling of the tungsten carbide-cobalt powder mixtures is usually performed in carbide-lined mils using tungsten carbide balls in an organic liquid to limit oxidation and minimize contamination of the mixture during the milling process. Organic lubricants, such as paraffins, are added to powder mixtures incident to milling to facilitate physical consolidation of the resulting composite powder mixtures. Prior to consolidation, the volatile organic liquid is removed from the powders by evaporation in for example hot flowing nitrogen gas and the resulting lubricated powders are cold compacted to form the intermediate density compact for subsequent sintering or mechanical compaction.
Prior to high-temperature, liquid-phase sintering, the compact is subjected to a presintering treatment to eliminate the lubricant and provide sufficient "green strength" so that the intermediate product may be machined to the desired final shape. Presintering is usually performed in flowing hydrogen gas to aid in the reduction of any residual surface oxides and promote metal-to-carbide wetting. Final high temperature sintering is typically performed in a vacuum at temperatures above about 1320.degree. C. for up to 150 hours with the compact being imbedded in graphite powder or stacked in graphite lined vacuum furnaces during this heating operation. In applications where optimum fracture toughness is required, hot isostatic pressing at temperatures close to the liquid phase sintering temperature is employed followed by liquid phase sintering to eliminate any residual microporosity.
As can be seen, with conventional practices, problems are encountered both in the synthesis and the blending of the powders. Specifically, kinetic limitations in the synthesis of the components require processing at high temperature for long periods of time. In additions, control of carbon content is difficult. Likewise, compositional control is impaired by the introduction of impurities during the mechanical processing of the composite powders and primarily during the required milling operation. Likewise, the long time necessary for achieving microstructural control and homogenization during milling ads significantly to the overall processing costs. Also, microstructural control from the standpoint of achieving desired hard-phase distributions is difficult.
Specifically, in various applications extremely fine particle dispersions of the hardening phase within a metal matrix is desired to enhance the combination of hardness, wear resistance and toughness.
Typically, particle reinforced materials are divided into two categories: dispersion strengthened and subparticle strengthened materials. Materials in these categories have the following properties:
______________________________________ Property Strengthened Strengthened ______________________________________ subparticle diameter &lt; 1 micron &gt; 1 micron matrix mean free path 0.01 to 0.3 micron &gt; 1 micron volume fraction of &lt; 0.15 &gt; 0.25 dispersoid ______________________________________ P. A. Thorton and V. J. Calangelo, Fundamentals of Engineering Materials, PrenticeHall, Englewood Cliffs, N.J. (1985) p. 593.
Preferably, the reinforcement is carried out with materials such as oxides of relatively low solubility within the metal matrix. In dispersion strengthened materials, the presence of foreign particles within the metallic structure inhibits the migration of dislocations within the matrix responsible for creep. However, in each case it is necessary to determine the optimum particle spacing and particle size for the given metal matrix before the particles can impart additional strength to the metal. These optimums vary from metal to metal. As a result, there is little ability to control the microstructure and homogeneity of the resultant metal powder during milling.
Moreover, the high surface area of the dispersed hard phase is generally unstable. Diffusive coarsening of the particle dispersion degrades the materials performance. Therefore, low solubility is necessary in order to kinetically limit thermal diffusive coarsening of the reinforcing dispersion.